There is a token-ring method (hereinafter simply called “token-ring”) for transmitting or receiving data between nodes (transmission stations) connected to a transmission line such as a LAN (local area network). Generally, in token-ring, a token is defined and constantly circles in a transmission line. This prevents frames from colliding each other on the transmission line. Namely, the token-ring differs from CSMA/CD method and can avoid the collision.
For example, Patent document 1 (Japanese Published Unexamined Patent Application No. 2004-166074) discloses a data transmission method in which a transmission station eligible to transmit a frame is controlled by a token, one transmission station is set as a synchronous node from among transmission stations, and a synchronous frame (SYN frame) including information as to whether or not a transmission station is active, which is transmitted by the synchronous node, is used.
Patent document 2 (Japanese Published Unexamined Patent Application No. 2008-131132) discloses a data transmission method in token-ring in which the token-ring has a ring topology where two or more transmission stations each of which has two communication ports allowing two-way communication are connected in a ring shape, any adjacent two transmission stations are set to be terminal stations, any transmission station is set to be a synchronous node, and transmission is controlled while a frame transmitted beyond the terminal station is set to be invalid.
These data transmission methods described in Patent documents 1 and 2 avoid frame collision by limiting to one the number of transmission stations (nodes) eligible to transmit a frame on a transmission line using a token for the predetermined duration. For example, a transmission station starts transmission to a master station in order of increasing a node number which is set as a token order. Thus, the length of time of transmission to the master station largely varies depending on the assignment of node number in the token-ring network system.
For example, there is a problem that a transmission time necessary for a synchronous time by a synchronous frame largely varies between a case where transmission stations are connected in a ring shape whose node numbers are assigned in order of transmission priority previously set and a case where transmission stations are connected in a ring shape whose node numbers are not assigned in order of transmission priority previously set. The latter case brings delayed accumulation between transmission stations, which causes delay of transmission time.
In the token-ring network system, there is a possibility that a transmission station is changed or added. In this case, it is necessary to create a loss-free network system where an operator determines a transmission priority of the transmission station and connects the transmission station on the transmission line in view of the transmission priority such that delay of transmission time does not occur. However, the operator is burdened with the creation of network system in view of a transmission priority.
Patent documents 1 and 2 disclose the methods in which the master station accepts an REQ frame from only a node which is allowed to transmit it by the master station, during MAC control time. Thus, in a case where 256 transmission stations (including a master station) enter into the transmission line, the master station needs to perform 256 scans using SYN frames. This requires a great deal of time until all transmission stations are allowed to transmit an REQ frame to the master station.